The present application claims priority to Japanese Application No. P2000-246023 filed Aug. 14, 2000, which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary cell capable of repeated charge/discharge.
2. Description of the Related Art
Recently, with development of various portable electronic apparatuses such as a video camera with video tape recorder, cellular phones, and portable personal computers, as a power source to drive these electronic, demand for secondary cells capable of repeated charge/discharge is increased instead of primary cells.
As such secondary cells, nickel-cadmium cells and nickel-metal-hydride cells are known, and special attention is paid on a so-called lithium-ion secondary cell, i.e., a non-aqueous electrolyte secondary cell using a cathode active material containing a lithium compound and an anode active material capable of doping/dedoping lithium.
This lithium-ion secondary cell has a high weight energy density and a high volume energy density and can play a great role for reducing the size and weight of portable electronic apparatuses.
Normally, the lithium-ion secondary cell includes an anode current collector containing Cu. In this lithium-ion secondary cell, when the cathode active material is a lithium-containing composite oxide such as a lithium-nickel composite oxide and a lithium-manganese composite oxide, the cathode potential prior to charge is approximately 3V against the lithium potential.
In the cell, lithium ions move from the cathode to the anode during charge and from the anode to the cathode during discharge. Because the cathode charge/discharge efficiency is 99% or above, if all of the lithium ions which have moved from the cathode to the anode during charge return to the cathode during discharge, the cathode potential against the lithium potential is approximately 3V. Accordingly, even if the cell voltage becomes 0V, theoretically the anode potential will not reach the dissolution potential of Cu contained in the anode current collector (3.45V against the lithium potential).
However, actually when lithium is doped in the anode active material, a film called Solid Electrolyte Interface (hereinafter, referred to as SEI) on the anode active material surface. If the SEI is formed on the anode active material surface, lithium capable of charge/discharge is consumed, decreasing the lithium ions returning to the cathode.
Accordingly, the cathode potential will not become less noble than the dissolution potential of Cu and the anode potential may reach the dissolution potential of Cu. Especially when the cell voltage is in an over discharge state such as 0.5 or below, Cu is dissolved out from the anode current collector. The Cu dissolved is deposited onto the anode during charge, significantly lowering the discharge amount.
For this, by providing an over discharge preventing circuit to prevent lowering the discharge amount. However, the present of the over discharge preventing circuit prevents disturbs to reduce the size and weight of portable electronic apparatuses. Accordingly, the lithium-ion secondary cell itself should have anti-over discharge characteristic.
It is therefore an object of the present invention to provide a non-aqueous electrolyte secondary cell preventing dissolution of Cu contained in an anode current collector, and having an excellent anti-over discharge characteristic and a high energy density.
The present invention provides a non-aqueous electrolyte secondary cell including: a cathode containing a lithium compound as a cathode active material; an anode having an anode current collector containing Cu, and a material capable of doping/dedoping lithium as an anode active material; and a non-aqueous electrolyte; the cathode active material containing a lithium-nickel composite oxide with a mixture ratio A/(A+B) in the range from 0.2 to 1 wherein A is assumed to be the weight of the lithium-nickel composite oxide and B is assumed to be the total weight of the cathode active material other than the lithium-nickel composite oxide, and the anode active material having a specific surface in the range from 0.05 m2/g to 2 m2/g.
By using the cathode active material containing the lithium-nickel composite oxide with the mixture ratio A/(A+B) in the range from 0.2 to 1, a cathode potential lowering speed is made faster during discharge, and a cathode potential is always less noble than a dissolution potential of Cu contained in an anode current collector, and then by using the anode active material having the specific surface in the range from 0.05 m2/g to 2 m2/g, it is possible to sufficiently suppress formation of SEI. Furthermore, this combination of the cathode active material and the anode active material enables to prevent dissolving out of Cu contained in an anode current collector and lowering of the discharge capacity even during over discharge.